Clinical allogeneic bone marrow transplantation is an important therapeutic treatment for several diseases including high risk leukemia, aplastic anemia, and severe combined immunodeficiency. In addition there is a wide range of metabolic and genetic disorders that can potentially be corrected by this approach. However, the usefulness of marrow transplantation continues to be limited by several important risk factors, the principal one being graft-versus-host disease (GVHD), an oftentimes lethal complication which occurs in a high proportion of transplants. The risk of GVHD can be reduced by HLA matching of the marrow donor and recipient, with a matched sibling being the primary choice. Yet, the risk of GVHD is still quite high due to disparity of non-HLA multiple minor histocompatibility antigens (miHA). In previous years we have clearly defined, in murine models, the relative etiological and pathological roles of both CD4+ and CD8+ T cell subsets in GVHD directed to miHA barriers. We have also found evidence for the oligoclonal, yet heterogeneous, nature of both T cell subset responses to miHA in vivo. The general aim of this current proposal is to continue our investigation of the immunobiology of lethal GVHD, with a focus on multiple miHA differences and how donor T cells develop the specific response to them. In this regard we will concentrate our efforts on the following specific aims: (1) the molecular and immunological analyses of the CD4+ T cell repertoire response to multiple miHA responsible for GVHD; (2) the molecular and immunological analyses of the CD8+ T cell repertoire response to multiple miHA responsible for GVHD; and (3) the utilization of the repertoire information to avoid GVHD and to retain optimal graft-versus-leukemia (GVL) responses. Insights generated from these studies on GVHD to miHA will hopefully lead to new approaches for overcoming one of the major obstacles for improved and expanded use of clinical allogeneic bone marrow transplantation.